The invention relates to equipment for measuring the high frequency current amplifying capability of bipolar transistors, which generally is a function of both the magnitude of the gain obtainable in the relatively low frequency range and the bandwidth or cut-off frequency, f.sub.c, thereof, which is the frequency at which the current gain drops 3dB from, or is 0.707 of, its generally constant maximum low frequency gain value. The most preferred form of the present invention provides test equipment for television and CB radio servicemen and others, to determine what is referred to as the f-tau (f.sub.t) of bipolar transistors, which is the frequency at which current gain drops to a value of one. This frequency f.sub.t is mathematically equal to the product of the low frequency gain and f.sub.c or bandwidth of the bipolar transistors under test.
Manufacturers list the high frequency capability of its transistors in terms of the f.sub.t value thereof. It has been determined mathematically that where the test frequency is much greater than f.sub.c (that is at least about three times greater than f.sub.c) f.sub.t is equal to the product of the test frequency and the gain at that test frequency. Transistor manufacturers have measured f.sub.t with test equipment which computes the product of such a test frequency and the gain of the transistor under test, but this test equipment is not easily useable with transistors of widely different types and is so sophisticated that servicemen cannot afford the same. Thus, such servicemen have not heretofore been able to test the f.sub.t (or gain-bandwidth product) of transistors. There are many instances when it would be helpful for servicemen to be able to test the f.sub.t of transistors. For example, most test equipment used by these servicemen measures the gain of transistors at relatively low frequencies, that is frequencies well below the frequency f.sub.c. If a transistor which is tested passes this gain test, and still the CB, television or other circuit involved does not operate properly, the serviceman has no way of knowing if the problem is due to a transistor which does not produce adequate gain at and well above the cut-off frequency. Additionally, there are many instances when the serviceman desires to know if he can substitute a high frequency transistor of one type, which may be a Japanese made transistor, with no well known replacement type of U.S. manufacture, with another transistor of a different type without a time consuming physical substitution of the latter transistor for the former transistor. It would, therefore, be extremely useful if servicemen could have modestly priced test equipment which could immediately give an indication of the high frequency capability of a substitute transistor by a quick out-of-circuit measurement.
One factor complicating the design of gain-bandwidth product measuring equipment useful to television and CB radio servicemen is that, unlike the test equipment used by transistor manufacturers where a particular piece of test equipment is set-up to test only one type of transistor, the test equipment for television and CB radio servicemen must be quickly and easily adaptable for testing transistors of a wide variety of types. The gain of transistors is generally tested under conditions where the transistor operates as a linear amplifier, and so the base-emitter circuit of a tested transistor includes a DC bias for producing a DC collector current which is varied in positive and negative going directions by the signal to be amplified applied to the base-emitter circuit thereof. Transistors of widely different types frequently produce a different DC collector current for a given DC bias applied to the base-emitter circuit thereof. Unfortunately, the current gain of transistors varies appreciably with the DC collector bias current, particularly at frequencies exceeding the cut-off frequency thereof. Therefore, the costly test circuits of the type heretofore utilized by transistor manufacturers are unsuitable for measuring the f.sub.t of transistors of different types which produce widely different DC collector current for a given base-emitter bias because either the DC base-emitter biasing voltages thereof are not readily adjustable, or, if adjustable, require the servicemen to make an adjustment thereof as different transistors are tested, which unduly complicates the measuring procedure.
It is, accordingly, one of the main objects of the invention to provide relatively inexpensive yet reliable and easy to use equipment for testing the f.sub.t or the gain-bandwidth product of bipolar transistors of a wide variety of types.